This invention relates in general to surface inspection systems, and in particular, to an improved system for detecting anomalies and/or features of a surface.
The size of semiconductor devices fabricated on silicon wafers has been continually reduced. The shrinking of semiconductor devices to smaller and smaller sizes has imposed a much more stringent requirement on the sensitivity of wafer or photomask inspection instruments which are called upon to detect contaminant particles and pattern defects as well as defects of the surfaces that are small compared to the size of the semiconductor devices. At the time of the filing of this application, design rule for devices of down to 0.13 microns or below has been in use or called for. At the same time, it is desirable for wafer inspection systems to provide an adequate throughput so that these systems can be used for in-line inspection to detect wafer and other defects.
One type of surface inspection system known as bright field illuminates a large area in a scheme sometimes known as flood illumination. High resolution images of illuminated areas of a surface are obtained from radiation reflected by the surface by means of two-dimensional imaging optics as the surface is scanned underneath the imaging optics. Such system requires significant time to image the entire surface of a photomask or semiconductor wafer because of the data rate required for imaging. For this reason, bright field inspection is typically used in back-of-the-line wafer processing systems, rather than in production.
In some bright field systems, radiation from a source is passed through a beam splitter towards the surface that is being imaged, and reflected radiation from the surface is passed through the beam splitter again before the reflected radiation is directed to a detector. Thus the radiation passes through the beam splitter twice between the source and the detector, so that the intensity of the radiation is much reduced upon reaching the detector. This greatly reduces the amount of photons originating from the source that reach the detector, and therefore reduces the sensitivity of bright field inspection. It is therefore desirable to provide an improved bright field system where such deficiencies are not present.
In another type of semiconductor inspection system known as a dark field system, instead of illuminating a large area of the surface inspected, the beam illuminates a small area or spot on the surface, where the spot is scanned across the surface. Instead of detecting reflected radiation, the detector is placed away from the specular reflection direction to detect scattered radiation. Hence if there is no anomaly on the surface, the image obtained from the detector will be totally dark. For this reason, such systems are known as dark field systems. The detector in dark field systems will provide an output only when one or more anomalies are present, in contrast to bright field systems. If the background wafer pattern is sparse or can be filtered out of the basic signal, the instantaneous pixel (inspection area) can be larger in dark field than in bright field while still maintaining the same detection signal capability and data rate is not as much a limitation for such systems. Dark field systems therefore typically have larger pixels and higher inspection throughput compared to bright field systems.
In one type of dark-field imaging, a laser spot is scanned rapidly across the wafer surface as the wafer moves beneath the scanning spot, and a signal-element detector receives the optical signal scattered from objects on the wafer surface. This signal is processed to produce a simulated two-dimensional image, which is then analyzed to locate and characterize wafer defects. Spot-scanning dark-field systems generally have higher inspection speed than bright-field systems, but with lower image resolution, and suffer some signal noise resulting from pattern on the wafer surface. Inspection throughput in dark-field systems, while generally higher than bright-field systems, is nonetheless limited by the rate at which the laser spot can be scanned.
The problems of scanned spot dark field systems are compounded when dark field systems are called upon to detect smaller and smaller defects. If the illuminated spot is large relative to the size of the defects to be detected, dark field systems will have low sensitivity since the background or noise signals may have significant amplitudes in relation to the amplitudes of the signals indicating anomalies within the spot. In order to detect smaller and smaller defects, it is, therefore, desirable to reduce the size of the illuminated area on the wafer surface. However, as the size of the illuminated area is reduced, throughput is usually also reduced. It is therefore desirable to provide a dark field system with adequate sensitivity but improved throughput.
While the above-described systems may be satisfactory for some applications, they can be inadequate or expensive for other applications. It is, therefore, desirable to provide an improved surface inspection system with improved sensitivity and performance at a lower cost that can be used for a wider range of applications.